[docs] Warn about empty directories in artifacts

[orensbruli]

What files would you like to change?

README.md

What are your suggested changes?

I think it would be worth mentioning the filtering of empty directories.

upload-artifact/src/search.ts

Lines 99 to 100 in 65d8626

	Directories will be rejected if attempted to be uploaded. This includes just empty
	directories so filter any directories out from the raw search results

I think by default, if we add a path, we expect to see it in the artifact that is generated, even if it is empty, another way we tend to think that something went wrong with the configuration of the action. Since this is not the case, I think it would be good to let the user know that the lack of empty directories is the expected behavior.

[orensbruli]

PR #393